Dental restorative material can provide a color close to the natural tooth and is easily operated in the treatment of tooth. Therefore, it has rapidly come into wide use as a restorative material for the teeth having caries, etc. In recent years, it has been used in most cases of the treatment of anterior teeth. Recently, it has begun to be used in the restoration of, for example, posterior teeth upon which a high occlusional pressure is applied.
Dental restorative material is a high viscosity paste-like composition constituted mainly by a polymerizable monomer, a filler and a polymerization initiator. The filler has a great influence on the esthetic properties, mechanical properties and the like of the cured body of the dental restorative material, depending upon the composition, shape, particle diameter, use amount, etc. of the filler used. For example, when there is used an inorganic fine filer made of inorganic particles of 1 μm or smaller in average particle diameter and/or an agglomerate thereof, the resulting dental restorative material can have good wear resistance and surface smoothness/gloss.
However, the inorganic fine filler has a very large specific surface area and, therefore, a paste-like dental restorative material containing such an inorganic fine filler has a high stickiness before curing. In order to allow this dental restorative material to have a stickiness suitable for operation thereof, the use amount of monomer in the material need be increased. In this case, however, there occur problems that the handling property of the dental restorative material deteriorates, the polymerization shrinkage amount of the cured body thereof increases, and the mechanical strength of the cured body decreases.
In order to avoid the above problems while realizing the high surface smoothness/gloss and wear resistance which are features when the above-mentioned inorganic fine filler is used, it was found to use an organic-inorganic composite filler. The organic-inorganic composite filler is produced by mixing beforehand an inorganic fine filler and a polymerizable monomer, polymerizing and curing the mixture, and grinding the polymerized resin obtained. This organic-inorganic composite filler contains an inorganic fine filler and therefore the dental restorative material using the composite filler can give, when cured, high surface smoothness/gloss and wear resistance. Further, since the inorganic fine filler is dispersed in the organic matrix constituted by the cured polymerizable monomer, the paste-like dental restorative material before curing has no excessively high stickiness.
Even in the dental restorative material containing the Organic-inorganic composite filler, there is a case that the color of the cured body thereof is unable to match the color of tooth. In such a case, patients are not fully satisfied when high esthetics is required in their dental treatments. Specifically explaining, it is such a case that dental restoration is conducted by selecting a paste-like dental restorative material (not cured) fully matching the color and transparency of the tooth of patient but the cured body of the material does not match the color and transparency of the tooth of patient. That is, it is a case that, after the actual restoration of tooth, the cured body of the dental restorative material filled in tooth does not accurately match the color and texture of surrounding tooth. Further, it is a case that the boundary between the surrounding tooth and the cured body can be recognized visually.
Meanwhile, the optical properties of the dental restorative material include a light-diffusing property which has a great influence on the esthetics of tooth. Here, the light-diffusing property is such a property of translucent material that a light incident on a translucent material (e.g. cured body of dental restorative material) is diffused into various directions in the translucent material. The diffusion of incident light takes place because the incident light is refracted and reflected by the filler contained in the translucent material.
The reflected light appearing due to the diffusion of a light incident on a translucent material has a color of the translucent material and a color reflecting the color of the background thereof. Accordingly, a dental restorative material of higher light-diffusing property makes smaller the difference between the color of the cured body of the dental restorative material and the color of the background and makes vague the boundary between cured body and tooth. That is, the color conformability between tooth and cured body is higher.
As an indicator of the light-diffusing property, there was proposed the diffusion degree (D) represented by the following formula. With a larger diffusion degree (D) of translucent material, the light transmitted through the translucent material shows higher light-diffusing property.D={(I20/cos 20°)+(I70/cos 70°)}/(2×I0)(in the above formula, I0, I20 and I70 are, when a light is applied perpendicularly to the surface of a sheet-shaped sample of given thickness obtained by curing a dental restorative material, the intensities of lights transmitted through the sample in the directions of 0°, 20° and 70° relative to the incident direction of the light applied.)
Under the above background, there was proposed a dental restorative material wherein the diffusion degree (D) of the above-mentioned dental restorative material containing the organic-inorganic composite filler was enhanced (see Patent Literature 1). In this dental restorative material, the diffusion degree (D) is enhanced, whereby the color conformability between the cured body of the dental restorative material filled in the cavity of tooth, i.e. filled material and the surrounding tooth is improved.
In order to practically enhance the diffusion degree of the cured body of dental restorative material, the Patent Literature 1 proposes, in claim 3, compounding, in a dental restorative material, an organic-inorganic composite filler satisfying the following conditions (1) and (2).                (1) The average particle diameter is 1 to 20 μm.        (2) The absolute value of nF−nM is 0.01 or larger. Here, nF is the refractive index of an organic-inorganic composite filler. nM is the refractive index of the matrix resin constituting the cured body of the dental restorative material        
In the Patent Literature 1, it is described that the dental restorative material may contain other inorganic filler (a third component inorganic filler) besides the organic-inorganic composite filler. Specifically, it is described that the dental restorative material contains fine spherical inorganic filler having an average particle diameter of 1 μm or smaller. The main purpose of adding this filler is to make easier the filling operation of uncured paste-like dental restorative material ([0020]). It is also described that other purpose is to allow the spherical inorganic filler to function as a controlling agent of the refractive index of matrix portion, to satisfy the above condition (2) for the organic-inorganic composite filler ([0065]). This fine spherical inorganic filler, however, has an average particle diameter close to the wavelength (0.4 to 0.7 μm) of visible light; therefore, the change of the refractive index of the matrix portion by the spherical inorganic filler is very small. That is, the influence on the diffusion degree of cured body is small when such a fine spherical inorganic filler is compounded in the dental restorative material.